Electrodynamics of Metallic Superconductors

Abstract

The theoretical and experimental aspects of the microwave, terahertz, and infrared properties of superconductors are discussed. Electrodynamics can provide information about the superconducting condensate as well as about the quasiparticles. The aim is to understand the frequency dependence of the complex conductivity, the change with temperature and time, and its dependence on material parameters. We confine ourselves to conventional metallic superconductors, in particular, Nb and related nitrides and review the seminal papers but also highlight latest developments and recent experimental achievements. The possibility to produce well-defined thin films of metallic superconductors that can be tuned in their properties allows the exploration of fundamental issues, such as the superconductor-insulator transition; furthermore it provides the basis for the development of novel and advanced applications, for instance, superconducting single-photon detectors.

THz spectroscopy of superconducting ultrathin films

“…The kink in obtain the frequency-and temperature-dependent superconducting properties such as the complex optical conductivity (Fig.2), the energy gap, the penetration depth, and the superfluid density. We discuss recent results obtained on granular Al and strongly disordered and NbN films pointing towards non-BCS behavior [1][2][3][4][5]. We have further studied the evolution of the superconducting energy gap of InO and NbN through the disorder driven superconductor-to-insulator transition (SIT) by comparing the results of optical THz to tunneling spectroscopy.…”

“…uasi-optical THz spectroscopy is a particularly suited probe to study superconducting properties of thin films, such as the energy gap or superfluid density, shedding light on current academic questions as well as understanding and improving the performance of novel devices such as single-photon detectors [1,2]. Raw data of (a) transmission Tr( ) and (b) relative phase shift ( )/ spectra of the Nb film on the sapphire substrate.…”

“…The blue lines correspond to the fit using the Mattis-Bardeen equation. In the insets the temperature dependence of Tr and / is displayed for two particular frequencies corresponding to the first and last transmission maxima in measured spectral range (after [2,3]). …”

London-Proca-Hirsch Equations for Electrodynamics of Superconductors on Cantor Sets

“…The relevance of the proposed new equations in lieu of (4)- (5) should be verified by experiments with superconductors [16]. For convenience, the equations (8a)-(8b) can be given a name: LondonProca-Hirsch equations.…”

Geometric scaling of two-level-system loss in superconducting resonators

“…The superconductor part of the structure requires extra care to simulate accurately: strongly disordered superconductors, like NbN, have an extremely small electron mean free path l (on the order of 0.5 nm and smaller [45]) and are therefore in the local dirty limit [46]. In this limit, several quantities become dependent on the mean free path and need to be adjusted from their BCS values [47,48].…”